Grounding Systems For the generator set
Grounding systems have specific characteristics that must be taken into account for protection against electric shock. Mobile devices cannot be connected to the earth, and their connection using a flexible cable can easily be damaged.
Generator Neutral Grounding Systems (photo credit: cat.com)
In general, generator sets have much lower short circuit levels than transformers ( around 3 × In instead of 20 × In ). As a result, the trip conditions required for protection against indirect contact cannot be provided by devices sized to operate on a normal supply.
- Portable generators
- Mobile generators for temporary installations
- Mobile generator set with fixed installation for one-off replenishment
- Mobile generator set for fixed installation for replenishment planned at the design stage
- Fixed assemblies for fixed installations
1. Portable generators
For temporary installations limited to a few kVA, these directly supply a small number of receivers (market stall, kiosk, power supply for portable tools, etc.).
The exposed conductive parts of the device and those of the installation must be connected to each other by a protective conductor. Each outgoing circuit must be protected by a residual current device i∆n ≤ 30 mA .
If the substation has one or more outlets without a protective DDR, there should be one RCD per circuit at a distance of less than 1 m. As earthing is not possible and the neutral pole is not accessible, the installation will operate as a computer system.
If the generator set supplies class II devices, the exposed conductive parts are not linked but the supply of one or more RCDs remains mandatory for additional protection against direct contact, in particular on the flexible connection cable.
By the way, Class II devices are devices where the exposed conductive parts of such devices do not need to be connected to a protective conductor.
2. Mobile generators for temporary installations
With powers greater than 10 kVA, these provide larger installations (construction sites, carousels, circuses, etc.). The exposed conductive parts of the device must be connected to the exposed conductive parts of the devices used by means of a protective conductor.
Protection against electric shock is provided by a residual current device i∆n ≤ 30 mA protecting all outgoing lines, generally incorporated into the set by construction.
If there are requirements for differential For discrimination between supplied circuits, secondary residual current devices i∆n ≤ 30 ma can be installed on each outgoing line, provided that they are at a distance of less than 1 m .
If there is a possibility of establishing a reliable earth connection, the installation can operate in TN-S system mode . The fault current is closed by the neutral or by connecting the exposed conductive parts if the neutral is not distributed. This is only possible for three-phase loads and allows the use of three-pole devices. In this case, it is a TN-S system with neutral neutral, not to be confused with a TN-C system.
If there are no grounding systems established, on the whole, the installation will operate as a grounding system. The protection and breaking devices must have an opening offset from the neutral with the protection of all poles. In addition, the neutral section must not be reduced.
NOTE! The installation and configuration of the generating sets are subject to specific regulations on the characteristics of the zones , the levels of emissions and pollutants of the exhaust gases and the admissible noise. It is advisable to refer to these regulations with the assistance of the manufacturers and the competent bodies.
3. Mobile generator set in grounding systems with fixed installation for one-off replenishment
Temporary one-off replenishment of a fixed installation instead of the mains supply or the usual supply must only be carried out after isolation.
Manual opening of the main circuit breaker generally ensures this separation, as long as it is held in position (locking, padlocking) or indicated by a warning sign.
In all systems (TT, IT, TN), the exposed conductive parts of the generator set must be interconnected with the earth network of the existing installation . If a local earth connection can be established for the neutral of the device, the earth must be interconnected with the equipotential bonding of the installation.
If, as often, this operation is impossible or not carried out, the installation will function as grounding systems if the neutral of the generator is not accessible.
If the generator neutral is accessible, it must be connected to the protection circuit of the fixed installation by a protective conductor (of identical cross-section) incorporated in the cable or by a separate cable, sized for the fault conditions, with a cross-section minimum copper diameter of 16 mm 2 . The installation will then function as a TN-S or TT system.
Important note: – In TN or IT systems, protection against indirect contact may not be provided. in installations to be replenished by a mobile generator set, a panel must be placed near the connection point, with the formulation
Minimum power of the assembly to be installed: x kVa.
In all systems (except TN-C) the provision of a residual current protection device is recommended. The toroidal residual current sensor must be placed downstream of the neutral point earth (see Figure 2) or on the generator neutral point earth conductor.
If the generator is a power supply for the security services, the earthing system used will be the it system.
4. Mobile generator set for fixed installation for replenishment planned at the design stage
When the replenishment of a fixed installation instead of the mains supply or the usual power supply is foreseen at the design stage an all-pole supply changeover switch must be installed .
Whatever the neutral earthing system of the fixed installation, it is necessary to interconnect the exposed conductive parts (TT, IT), the neutral point of the assembly and the exposed conductive parts of the assembly (TN ) to the exposed conductive parts of the existing installation.
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If the overcurrent protection devices (IT and TN systems) do not meet the protection conditions (see Figure 5), a high sensitivity residual current device (30 mA) must be used and the neutral grounded in upstream of the differential (see the diagrams below).
In TT systems, an RCD must be used in all cases. The part upstream of the residual current device must have double or reinforced insulation. The toroidal sensor must be placed on all live conductors (phase + neutral) or on the conductor connecting the neutral point of the alternator to the earth of the installation (TT or TN-S).
This solution is not applicable in TN-C systems .
When a generator provides autonomeinstallation without outlet or whose business continuity is paramount (machine, crane, carousel), it is permissible not to install differential device as the conditions for protection against indirect contact are filled in conformity with the chosen neutral earthing system.
5. Fixed assemblies for fixed installations
If it is a replacement power supply, it must use the same neutral grounding system as the normal power supply.
The conditions of protection against indirect damage, contact and tripping for minimum short circuits must be verified and must be observed whenever the installation is supplied by the normal power supply and by the generator set.
The security installations should preferably be created with the computer systems or under the conditions of TN systems.
5.1 Conditions of protection against short-circuits and indirect contact
The setting or value of the overcurrent Protection devices providing protection against indirect contact when using a neutral earthing system for a generator set must be chosen with care. the low value of the fault current is not always compatible with the fuse breaking time.
The current rating of these fuses and that of the generator must be similar and it is essential that the trip conditions are verified .
Likewise, if circuit breakers are used, the magnetic operation setting (short delay) should be set to a low threshold.
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